1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an ink ejection control method. Particularly, the present invention relates to controlling of pulses used in an ink ejection system in which the pulses are applied to an electro-thermal transducer element to heat ink and to cause a bubble for ejecting ink.
2. Description of the Related Art
Inkjet printing apparatuses are so-called non-impact type printing apparatuses that perform the high speed, reduced-noise printing for various types of print media. Because of the evident advantages afforded by such inkjet printing apparatuses, they are widely employed as printing mechanisms in printers, copiers, facsimile machines and large format printers for industrial use (the printing of posters, CAD graphics, etc.).
Conventionally, dye ink has been employed for inkjet printing apparatuses; however, printed matter with dye ink is generally inferior in lightfastness, gas resistance and water resistance, and is not appropriate for outdoor posted notifications or for records to be kept for a long period.
On the other hand, a pigment ink employing a pigment as a coloring agent is superior to dye ink in lightfastness, gas resistance and water resistance. However, the pigment used is not dissolved in a solvent, but is dispersed in a solvent. Therefore, because the viscosity of a solvent used as a dispersing agent is high, the viscosity of pigment ink tends to be higher than that of dye ink. Furthermore, especially for some types of business-use printing apparatuses that provide superior ink fixing properties and color development while performing fast, high-quality printing, a surface preparation process is performed for paper to react with ink. In this case, a solvent that reacts with a paper surface preparation fluid may be added to ink, and accordingly, the viscosity of the ink tends to become higher than that of normal pigment ink. Furthermore, in a low temperature environment the viscosity of such high-viscosity ink is sharply increased, and as a result, a problem may arise in that the amount of ink ejected could be reduced or deterioration of the ink refill performance could occur.
As a measure to this problem, there is a well known conventional inkjet printer or the like that adjusts the temperature of ink (hereinafter also referred to simply as temperature adjustment) to control ink viscosity that exercises an affect on the volume of ink ejected.
For instance, an example inkjet printer of this type performs temperature adjustments by employing a heater (either a heater used only for heating a print head, or a heater also used for an ink ejection) for heating the print head where ink is held, and a temperature sensor for detecting the temperature of the print head associated with the ink. Specifically, the temperature adjustment is performed by feeding back the temperature detected by the temperature sensor to an application of heat by the heater. Also, there is known one in which detected temperature feedback is not employed, and instead, heat generation using a heater is simply controlled to perform the temperature adjustment. One arrangement for this unit provides for the heater and a temperature sensor to be mounted near the print head, e. g., to be mounted on the member constituting the print head, whereas another arrangement provides for the heater and the temperature sensor to be mounted separately from the print head.
Furthermore, a system has been known that directly changes the amount of ink to be ejected, without performing a temperature adjustment. This system may be employed separately, or with one of the above described methods. Specifically, according to this system, upon the application of a pulse to an electro-thermal conversion element (hereinafter also referred to as an ejection heater), thermal energy is generated by the ejection heater to heat ink and form a bubble, and the pressure built up by the bubble is employed to eject ink. In this system, a pulse width of the pulse (hereinafter also referred to as a heat pulse) to be applied to the ejection heater is changed to control the quantity of heat generated for changing the volume of the ejected ink.
The following performance manners of the temperature adjustments, which are made by employing and combining the above described arrangements, are known.                (1) Constantly performing temperature adjustments for a print head (outside/vicinity). Temperature feedback included.        (2) Performing temperature adjustments for a print head only as needed (outside/vicinity). Temperature feedback included.        (3) Performing temperature adjustments for making a print head at a high temperature (higher than that of an environment). Temperature feedback included.        (4) Modulating the pulse width of a single heat pulse (single pulse).        (5) Modulating the pulse width of divided heat pulses (double pulse).        
However, in the performance manner (1), since temperature adjustments are constantly performed, the evaporation of a solvent water in ink, which is accompanied by heating by a heater, is accelerated and thus induce increasing viscosity of ink inside an ejection opening of a print head, or even adhesion of ink to the ejection opening. As a result, an ejection malfunction, such as a deflection of ink in which an ink ejection direction is deviated and an ink ejection failure, may occur, or relative increase in the concentration of a coloring agent in ink may occur to cause a density change or uneven density in a printed result. In any event, quality deterioration of a printed image would occur.
In the performance manner (2), temperature adjustments are performed only as needed, i.e., this is an improved version of the performance manner (1). According to this performance manner (2), a temperature adjustment is initiated, for example, only after a printing instruction has been entered. Therefore, energy for heating (e.g., the heating quantity ((W) for the heater) must be supplied to reach a predetermined temperature within a comparatively short period of time. But in the event the width of a temperature ripple is increased in a temperature control, a situation may be encountered wherein the accurate temperature control is not possible. In such a case, the amount of ink ejected may fluctuate, due to the ripple, and a density change, or unevenly densities, may occur. On the other hand, to accurately perform a temperature adjustment, the energy to be provided must be reduced. Accordingly, an extended period will be required to reach the target temperature, and thus a problem arises, such that there is an increase in the waiting period for the printing start.
The performance manner (3) is relative for a system wherein a temperature to be adjusted is set that is higher than that for the surrounding environment, in order to counter an effect produced either by a local, external temperature change, or by one that occurs as a result of an increase in the temperature of a print head (a temperature increase occurring during printing). According to performance manner (3), during low-duty printing, fluctuations in the ejected ink volume can be reduced. However, during high-duty printing, e.g., during so-called solid-paint printing, an effect produced by a temperature rise can not be avoided. In addition, a satisfactory temperature adjustment response for a temperature rise can be obtained when a heater or a temperature sensor is mounted on a substrate, such as one made of alumina, that supports a heater board whereon an ejection heater is arranged. However, the heat capacity of the alumina material used for the substrate is quite large, and thus a temperature ripple could appear that would cause the ejected ink volume to fluctuate.
A system provided in accordance with the performance manner (4), for which the modulation of a pulse width can be accomplished using a single heat pulse (hereinafter also referred to as a single pulse), is employed for a bubble formation, an ink ejection method. Specifically, according to this system, the amount of ink to be ejected can be altered by changing the pulse width of the single pulse. This system, however, can not provide a change, in the amount of ink ejected, that can counter a fluctuation that occurs as a result of a temperature change at a print head. Therefore, a problem, in this case, is that a pulse width modulation system uses the single pulse to control the amount of ink ejected at only a small control width.
A system set up in accordance with the performance manner (5), as described in Japanese Patent Laid-Open No. H05-031905 (1993), modulates pulse width using a divided heat pulse and does not have the problems encountered by systems set up in accordance with the above described performance manners. According to an ink ejection control sequence employed for this system, at predetermined periodical intervals, a pre-heat pulse is supplied to heat ink, but only to a temperature whereat the ink is not ejected, and to thereafter supply a main heat pulse that is employed to eject the ink. In this instance, the pulse width of the pre-heat pulse is controlled in order to maintain a constant quantity of ink to be ejected. In a low temperature environment, for example, the pulse width that is set is greater than the pulse width that is used at a normal temperature. Then, when the control sequence is performed at the low temperature, the amount of ink ejected is prevented from being reduced and a stable amount of ink can be ejected.
In addition, the application of the pre-heat pulse is performed to raise the temperature of the ink around the ejection heater, and also to reduce the viscosity of the ink. Furthermore, as a result of the heating performed using the pre-heat pulse, a desired amount of ink can be ejected when the main heat pulse is applied.
However, when the system in accordance with the performance manner (5) is simply performed in case of employing a more viscous ink, such as the above described pigment ink, ink refill function may be deteriorated in a low temperature environment.
Specifically, the rate at which viscosity rises is increased in certain low temperature environments. And in such a case, when the viscosity of ink can not be appropriately reduced by extending the pre-heat pulse width, a fluid velocity of the ink will be lowered. And should this occur, a longer period would be required to fill (or refill) the ink paths inside the ejection openings of the print head and to prepare for the ejection of ink. As a result, the ink refill operation could not be synchronized with the ejection cycle, and this in turn could cause an ink ejection malfunction, such as less ejection amount than that specified for ejection. Especially when high-duty printing is being performed, the degradation of print quality, due to refill failures, becomes overly conspicuous.
Further, even when the pre-heat pulse width is to be extended in order to reduce the viscosity of ink or to maintain an ejection amount, there is a limitation (a required refill time period) on the pre-heat pulse width, as a consequence of an increase in printing speed. Specifically, when there is an increase in printing speed, this is accompanied by a corresponding shortening of the length of the print head drive cycle, and the length of the period for the application of a heat pulse including the pre-heat pulse, must not exceed the length of the cycle.
Moreover, in addition to a demand for increased printing speeds, there is a like demand for a high image quality. That is, a demand exists for improvements in all printing capabilities that would ensure the ejection amounts of ink during printing, even in low temperature environments.